1. Field of the Invention
This invention concerns a joint replacement system including a prosthesis having a novel exterior surface configuration for improved fixation within the surrounding bone. In preferred embodiments, the system includes a femoral prosthesis and a complimentary acetabular component which optimize the biological fixation of the bone during post-operative osteogenesis.
2. Description of the Prior Art
Many patients have enjoyed the benefits of joint replacement surgery wherein an artificial joint is substituted for a degenerate or damaged biological joint. This type of surgery is particularly prevalent in the hip joint, where often the preoperative patient experiences substantial pain in even the routine task of walking. The hip joint replacement operation is typical of joint replacement operations, in that the bone comprising the existing joint is removed and a hip replacement system including a femoral component and an acetabular cup (together with a friction-resistant insert) are substituted. Such surgery is commonly referred to as total hip arthroplasty.
This type of surgery requires a great deal of time in the operating theater. Before the surgeon can begin the process of implanting the replacement components, he or she must first make a posteriorlateral incision, retract or dissect the covering musculature, dislocate the hip, and remove the femoral head. The acetabulum must also be reamed out to receive the acetabular cup, and the femur drilled and reamed to receive the femoral component. The greater the amount of bone removed from the medullary bone space, the more effort will be required and the more stress placed on the surrounding bone during surgery. Therefore, prostheses which "fill the medullary bone space" may cause femoral crack or complete fracture during insertion. When the prosthesis fills the medullary bone space, the surgeon loses the margin for sizing error. In addition, when little or no cancellous bone is left, blood vessels are lost and bone regeneration to the prosthesis is limited.
One trend which has developed is the use of a "custom" cementless prosthesis, where biological fixation is employed to hold the prosthesis in place. Custom implants allow the surgeon to tailor the prosthesis to the desired length, diameter, and curvature of the patient to enhance the fit and function of the joint replacement. Uncemented implants appear preferable in patients placing more stress thereon, such as heavier patients, those which are more active and younger patients which will have an implant over a greater length of time. In order to be successful, uncemented implants must achieve long-term fixation necessary for a pain-free function and positive bone growth around the prosthesis. One preferred method of biological fixation at present is a coating of hydroxyapatite (Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2) which enhances bone fixation to the prosthesis as opposed to the uncoated surface of titanium or stainless steel.
Another problem associated with a cementless prosthesis is stress-shielding. This phenomena occurs when the bone fixes biologically to the tip of the prosthesis shank but the body or proximal portion of the prosthesis remains loose. Over time, the bone may wither away from the proximal portion, leading to weakening and eventual failure of the prosthesis due to loss of bone support and resulting metal fatigue. These problems are not generally encountered in cemented prostheses where the proximal or body portion is initially cemented to the surrounding bone.
One of the most significant stresses encountered by a cementless total hip arthroplasty patient post-operation is climbing stairs. This exercise concentrates the stress at the end of the prosthesis because of the stress riser or concentration developed at that point. This occurs for two reasons: the bone is designed to flex slightly while the prosthesis itself is more rigid, and the cavity created by the surgeon may be larger than the prosthesis, causing some stress concentration.
Typical acetabular components affix to the pelvic bone by pins or screws which are inserted therein. The acetabular component thus relies on the pins to hold it in place. This requires a greater acetabular component "shell thickness" to hold the pins or screws, thereby either requiring more removal of the patient's bone or a smaller thickness of the ultra-high molecular weight polyethylene (UHMPE) insert which has some cushioning properties. The screws are both expensive (being made of titanium or less advantageously of stainless steel) and subject to corrosion.
There has thus developed a need for an improved joint replacement system requiring less time, effort and expense during surgery, providing improved biological and mechanical fixation, creating less stress on the surrounding bone, and leaving a portion of the endosteum or inner cortex of the bone intact to promote the flow of blood and biologic fixation therewithin.